The 42 has a bigger battery and theoretically should last longer. If you've just recently gotten yours it should get better. You might want to shut it down, start it and put it on the charger. My wife has a 38 and the first day or so I noticed that her remaining charge was significantly less than mine but it has since improved.i have a 38mm sports model and my battery goes after 15hours and 30minutes in power save mode. Normal use.
Anybody else this short?
How long does a 42mm Holding up?
Two things you can also do that improved the battery time on mine is enable Reduce Transparency, that will make everything easier to read as well, and Reduce Motion, that won't improve legibility but one of the tech reviewers said that it improved battery life. The Reduce Transparency made a noticeable difference on mine and I have enabled it on my wife's.